Wastewater treatment lagoons are one of the most broadly used wastewater treatment technologies in the United States. Lagoons as a treatment technology are suited for small to medium-sized rural communities, animal feedlot operations, food production operations, as well as some general industries. The primary advantages of lagoons are low cost and ease of operation. Generally speaking, lagoons can be effective at removing organic material and suspended solids, provided the lagoons are not overloaded.
One disadvantage of most types of lagoon systems is their inability to remove some undesirable compounds, such as carbon, nitrogen, and phosphorous containing compounds and other undesirable substances from wastewater streams. In addition, it can be difficult to reduce biochemical oxygen demand (BOD) and total suspended solids (TSS) in wastewater lagoons. It is these undesirable compounds that increase the stench and subsequent neighbor complaints from lagoon systems. Undesirable compounds are removed from wastewater streams in lagoons by bacteria that break down such compounds.
The breakdown of these compounds is inhibited by several biological factors inherent in the design of an open lagoon. It is also known that the waste secretions from certain strains of algae can reduce growth of desirable bacteria.
Further, trickling filters are one of the oldest forms of wastewater treatment. Rocks or other media are stacked in a basin and wastewater is trickled over the media. A thin bio-film consisting of micro-organisms develops on the surface of rocks and other substrates and is able to remove the organic material out of the wastewater as it flows past the surface. The cross-sectional thickness of the bio-film provides conditions suitable for aerobic bacteria at the free surface, and in some cases, anaerobic bacteria near the less oxygenated substrate zone. Trickling filters are effective at removing compounds from wastewater due to the diversified zones favorable to the growth of different types of micro-organisms that break down organic matter. The fixed media provides a surface for bacteria to attach (the bacteria bio-film remains in place until it becomes so thick that it sloughs off). Oxygen requirements for various bacteria colonies will be met at some point across the thickness of the bio-film.
The primary disadvantages of a trickling filter are the initial capital costs to build the filter, pumping costs to lift the wastewater to the top of the filter, maintenance of the mechanical distribution system at the top of the filter, and ultimate disposal and replacement of the media within the filter.
Other designs that provide surface area for fixed film growth are Rotating Biological Contractors (RBCs), and various designs that place foam blocks and spacers or fibrous material down in the wastewater. RBCs require mechanical rotation systems. Capital costs to reach the equivalent surface area of a trickling filter can be quite high.